Low-density polyethylene (LDPE) produced by a high-pressure radical method is well known as a branched polyolefin, and the side chain of the LDPE has a non-linear tree-like structure. Such a structure is excellent in moldability and advantageous in melt processing, but on the other hand, it has a disadvantage of lowering mechanical strength of the polymer.
In the use application where mechanical strength is necessary, therefore, linear low-density polyethylene (LLDPE) obtained by copolymerization of ethylene and a higher α-olefin is generally employed. However, the length of the side chain of the LLDPE is extremely shorter than that of the LDPE, so that the LLDPE does not have good moldability that is an advantage of the LDPE.
On this account, development of branched polyethylene which is superior to the LDPE in mechanical strength and melt processability has been made enthusiastically. For example, a macromonomer produced by the use of a metallocene catalyst is copolymerized with ethylene to obtain a branched polymer satisfying the above-mentioned properties, as described in National Publication of International Patent No. 502303/1996. The polymer obtained by the process described in this publication has a structure similar to that of LLDPE, and its side chain does not become completely linear and has a methyl branch. Although the polymer of this structure is superior to LDPE in mechanical strength, the methyl branch lowers mechanical strength, and therefore the mechanical strength of the polymer is sometimes unsatisfactory depending upon the purpose.
In Japanese Patent Laid-Open Publications No. 316711/1998 and No. 38418/2000, a process for preparing a branched polymer by copolymerizing a macromonomer and an α-olefin is disclosed, but a method of inhibiting formation of methyl branch of the side chain has not been known yet. For the invention described in the above publications, it is difficult to control the weight-average molecular weight of the side chain to be in the range of 1,000 to 10,000, and there is a limitation on the process for preparing a branched polymer having a main chain and a side chain different in the composition.
In National Publication of International Patent No. 502303/1996, there is disclosed a branched polyolefin comprising: a main chain (b) of a homopolymer or a copolymer of C2–C30 alpha olefins; and side chains (a) of at least 250 carbon atoms comprising a homopolymer or a copolymer of C2–C30 alpha olefins, said side chains being distributed along the polymer main chain at an average frequency of 0.1 to 5 side chains per 1000 main chain carbon atoms, said branched polymer having a weight average molecular weight of at least 30,000 and an MW/Mn of 6 or less.
In view of such prior art as described above, the present inventors have found novel polymers having equivalent or smaller melt tension (MT (g)) and larger flow activation energy value than conventional polymers having equivalent recurring unit of the main skeleton and substantially the same molecular weight, molecular weight distribution and crystallinity as those of the novel polymers. The present inventors have further found a branched polyolefin of novel structure among the above polymers, said polyolefin mainly containing mainly ethylene units and a side chain composed of an olefin chain having few methyl branches, and a process for preparing the branched polyolefin. The present inventors have studied, as the branched polyolefin, a branched polyolefin containing scarcely any methyl branch and having a specific weight-average molecular weight, and as a result, they have found that the desired branched polyolefin can be obtained by the use of a catalyst containing specific two different transition metal compounds. Based on the finding, the present invention has been accomplished.
That is to say, it is an object of the present invention to provide a novel polymer having characteristic melt tension and flow activation energy which are not observed in the conventional polymers, and in particular, it is an object of the invention to provide a branched polyolefin having excellent moldability and mechanical strength and a process for preparing the same.